This adds support in dsymutil for mergeable libraries.
dsymutil reads a new stab emitted by ld, allowing it to operate on dynamic libraries instead of object files. It also now loads the Dwarf files associated to the libraries, and build the debug map for each binary from the list of symbols exported by the library.
For each Debug Map Object, there is a new associated Relocation Map which is serialized from the information retrieved in the original debug_info (or debug_addr) section of the .o file.
The final DWARF file has multiple compile units, so the offsets information of the relocations are adjusted relatively to the compile unit they will end up belonging to, inside the final linked DWARF file.